1. Field of the Invention
The invention relates to improvements in matched pre-emphasis and de-emphasis circuits for broad-band signals, particularly video signals, intended to be passed through a television signal transfer channel.
2. The Prior Art
Pre-emphasis and de-emphasis circuits are known for improving the signal-to-noise ratio in wide band signal transfer circuits. For example, U.S. Pat. No. 3,288,930, issued Nov. 29, 1966, describes circuits for pre-emphasis and de-emphasis of video signals. These circuits utilize diodes in a non-linear frequency-disciminating portion thereof for attenuating or accentuating the amplitudes of high-frequency, high-amplitude signals. Although the circuits disclosed in U.S. Pat. No. 3,288,930 are somewhat effective for the intended purpose of improving the signal-to-noise ratio in the overall system, there are some disadvantages of such circuits which result in a less than desirable overall frequency-response characteristic. The frequency response characteristic given in FIG. 7 of the U.S. Pat. No. 3,288,930 shows that the circuit is both frequency and amplitude sensitive, such that lower frequency signals are subjected to a substantially constant gain, while relatively higher frequency signals are subjected to a gain which is inversely related to input signal amplitude.
Although the circuit of the U.S. Pat. No. 3,288,930 serves its purpose reasonable well, it has been found that the lower break-point frequency of the frequency-response characteristics (i.e. at about 100-500 KHz in the U.S. Pat. No. 3,288,930 FIG. 7) is variable with signal amplitude.
Since the break-point of the frequency-response characteristic of the pre-emphasis circuit of the U.S. Pat. No. 3,288,930 changes with the signal level, it is necessary to provide a de-emphasis circuit which is very closely matched to the characteristic of the pre-emphasis circuit. This of course necessitates careful matching of components so that the time-constant, and hence the lower break-point frequency, of the de-emphasis circuit is as close as possible to the time-constant (and lower breakpoint frequency) of the pre-emphasis circuit.